Method of firing explosive charges and apparatus therefor



April 1957 K. J. BRIMLEY ETAL 2,788,461

EETHOD 0F FIRING EXPLOSIVE CHARGES AND APPARATUS THEREFOR Filed Oct. 28, 195:5 2 Sheets-Sheet 1 Jamaagaq%id (lance, BY m M7 0 ATTORNEYS.

April 1957 K. J. BRlMLEY ETAL 2,788,461

METHOD OF FIRING EXPLOSIVE CHARGES AND APPARATUS THEREFOR 2 Shets-Sheet 2 Filed 061:. 28, 1953 INVEN TORS.

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METHOD OF FRENG EXPLGSIVE CHARGES AND APPARATUS THEREFGR Kenneth .lohn Erirnley, tevenston, and .lames (Iraig Reid (lance, Saltcoats, Scotland, assignors to Imperial Chemicai Industries Limited, a corporation of Great Britain Application October 28, 1953, Serial No. 338,838

Claims priority, application Great Britain November 28, 1952 1 Claim. (Cl. 313-4173) The present invention relates to an improvement in the art of delay blasting and especially short period delay blasting in quarries and other situations where it is required that a single setting-up of the wiring of a succession of electric detonators will result in the detonation of the explosive charges successively at time intervals which may be as low as only a millisecond apart, when the shotfirer performs a single manual operation.

Short period delay blasting has in certain circumstances advantages that are well recognised over simultaneous blasting, as occurs when instantaneous electric detonators are used in the normal manner. Among these advantages are reduction in the amount of backbreak (i. e. the undesirable cracking of rock which occurs behind and is caused by the firing or blasting of a row of vertical shot holes) and of vibration, and facilitation of fragmentation of the rock to the desired size. Hitherto short period delay blasting has been carried out either by using a graded series of short period delay electric detonators, or by firing each of a sequence of instantaneous electric detonators in a properly wired shot-firing circuit through a sequence switch mechanism.

In practice, however, it is difiicult in repetitive manufacturing operations to produce short period delay electric detonators whose individual delay periods are sufficiently close to their respective average delay periods to obviate the possibility that in a round the detonators will not always fire in the desired order, especially when the time interval required between the successive shots in the round is relatively small, as is frequently desirable. When the number of shots in the round is large and the delay period Of the slowest 'detonator is relatively long, either the lengths of some of the delay elements and consequently of the short period delay electric detonators containing them tend to become excessive, or a variety of delay compositions have to be used in the series, which adds to the difiiculty and expense of the production of the short period delay electric detonators. On the other hand the use of a sequence switch mechanism has required heretofore that each electric detonator in the round is to be wired to a separate contact of the sequence switch mechanism so that the total length of leading wire required and the inconvenience of handling it increases considerably with the number of shots to be fired. Also the accommodation of the required number of switch contacts in the sequence switch mechanism be comes increasingly inconvenient. In practice the number of successive intervals that has been obtained by using electric detonators with a sequence switch mechanism has not substantially exceeded 16, and using short delay period detonators the number of successive intervals attained has been much the same. It would, however, be desirable to provide a short delay blasting method and equipment that would enable a considerably larger number of delay intervals to be obtained in a round.

In United States application Serial No. 352,128 there is described and claimed inter alia a method of firing a succession of shots by the firing of their corresponding 2,788,46l Patented Apr. 9,

electric detonators with a delay period between successive shots which comprises arranging that an electric firing current is passed through the fusehead of each succeeding detonator by way of an electrically actuable delay switch which is interposed between it and its preceding detonator so as to permit the electric firing current to pass through said fusehead when a closed circuit is formed by the actuation of said switch.

United States application Serial No. 352,128 also describes and claims inter alia a layout for firing a succession of shots by the firing of their corresponding electric detonators with a delay period between successive shots comprising an electrically actuable delay switch which is interposed between the iusehead of each succeeding detonator and its preceding detonator and which is adapted to form the closed circuit required after the preceding detonator has fired thereby to permit the electric firing current to pass through the fusehead of the succeeding detonator.

In United States application Serial No. 352,128 there is also described and claimed inter alia an electrically actuable delay switch comprising a tubular body containing within it an electric fusehead, two terminals at least one or" which is insulated from the tubular body, and a metal partition element positioned between said fusehead and terminals and adapted to be displaced to form a metallic bridge at least momentarily between said terminals on creation of a gas pressure on that side of it facing said electric fusehcad, wherein said fusehead comprises an electric resistance wire and a combustible composition element adapted to ignite to yield hot gases when an electric current is passed through said wire.

The object of the present invention is to provide an improved and simplified short delay blasting method and equipment by which the possibility of the shots firing out of their proper order is eliminated and which is capable of operation so as to fire any desired number of shots in a round.

According to the present invention the method of firing a succession of shots by the firing of their corresponding electric detonators with a delay period between successive shots comprises providing electrically actuable delay switches each comprising a container having Within it an electric fusehead comprising an electric resistance wire and combustible composition element adapted to ignite to yield hot gases when an electric current is passed through said wire and two terminals at least one of which is insulated from the container and so distanced from each other and the fusehead that on combustion of said combustible composition the space between the terminals becomes a conducting path for the dillerence of potential available between the terminals, and arranging that an electric firing current is passed through the fusehead of each succeeding detonator by said one of said electrically actuable delay switches which is interposed be tween it and the preceding detonator so as to permit the electric firing current to pass through said fusehead when a closed circuit is formed by the actuation of said switch.

In accordance with one embodiment of the invention the method of firing a succession of shots by the firing of their corresponding electric detonators with a short delay period between successive shots comprises providing electrically actuable delay switches each comprising a container having Within it an electric fusehead comprising an electric resistance wire and combustible composition element adapted to ignite to yield hot gases when an electric current is passed through said wire and two terminals at least one of which is insulated from the container and so distanced from each other and the fusehead that on combustion of said combustible composition the space between the terminals becomes a conducting path for the difference of potential available between the terminals,

connecting one lead wire of the first detonator to be fired to a common conducting element connectable to one pole of a source of electricity and the other lead wire to a leading wire of the electric fusehead of one of said electrically actuable delay switches and the other leading wire of said electric fusehead to another common conducting element connectable to the other pole of the source of electricity, connecting one lead wire of the next detonator to be fired to the common conducting element connectable to the first mentioned pole of the source of electricity and the other lead wire to the leading wire of the electric fusehead of a second electrically actuable delay switch as well as connecting one lead wire of one of the terminals of the first electrically actuable delay switch to the other leading wire of the electric fusehead of the second electrically actuable delay switch and the leading wire of the other terminal of the first electrically actuable delay switch to the common conducting element connectable to the other pole of the source of electricity, and so on, and thereafter permitting an electric current to pass from said source of electricity through said first detonator and electric fusehead of said first electrically actuable delay switch and then through said second detonator and electric fusehead of said second electrically actuable delay switch, and so on.

More particularly in accordance with another embodiment of the invention the method of firing a succession of shots by the firing of their corresponding electric detonators with a short delay period between successive firings comprises providing electrically actuable delay switches each comprising a container having within it an electric fusehead comprising an electric resistance wire and combustible composition element adapted to ignite to yield hot gases when an electric current is passed through said wire and two terminals at least one of which is insulated from the container and so distanced from each other and the fusehead that on combustion of said combustible composition the space between the terminals becomes a conducting path for the difference of potential available between the terminals, connecting one lead wire of the first detonator to be fired to a common conducting element connectable to one pole of a source of electricity and the other lead wire to a leading wire of an electric fusehead of one of said electrically actuable delay switches and the other leading wire of said electric fusehead to another common conducting element connectable to the other pole of a source of electricity, connecting the leading wire of one of the terminals of said switch to one of said common conducting elements and the leading wire of the other terminal of said switch to a lead wire of the second detonator to be fired, connecting the other lead wire of the second detonator to a leading wire of the electric fusehead of the second electrically actuable delay switch to be fired and the other leading wire of said electric fusehead of said second delay switch to the other common conducting element, and so on, and thereafter permit-ting an electric current to pass from said source of electricity through said first detonator and electric fusehead of said first electrically actuable delay switch and then through the terminal portion of said first delay switch, said second detonator and electric fusehead of said second electrically actuable delay switch, and so on.

The method of the present invention enables a round of any desired number of shots to be fired with current of an order sufficient only for the firing of two electric fuseheads at a time.

A layout for firing a succession of shots by the firing of their corresponding electric detonators with a delay period between successive shots according to the invention comprises an electrically actuable delay switch comprising a container having within it an electric fusehead comprising an electric resistance wire and combustible composition element adapted to ignite to yield hot gases when an electric current is passed through said wire and two terminals at least one of which is insulated from the container and so distanced from each other and the fuse head that on combustion of said combustible composition the space between the terminals becomes a conducting path for the difference of potential available between the terminals, which switch is interposed between the fusehead of each succeeding detonator and its preceding detonator and which is adapted to form the closed circuit required after the preceding detonator has fired thereby to permit the electric firing current to pass through the fusehead of the succeeding detonator.

More specifically, a layout according to one embodiment of the invention for firing a succession of shots by the firing of their corresponding electric detonators with a delay period between successive shots comprises electrically actuable delay switches each comprising a container having within it an electric fusehead comprising an electric resistance wire and combustible composition element adapted to ignite to yield hot gaseswhen an electric current is passed through said wire and two terminals at least one of which is insulated from the container and so distanced from each other and the fusehead that on combustion of 'said combustible composition the space between the terminals becomes a conducting path for the difference of potential available between the terminals, and is one wherein one lead wire of the first detonator to be fired is connected to a common conducting element connectable to one pole of a source of electricity and the other lead wire is connected to a leading wire of the electric fusehead of one of said electrically actuable delay switches and the other leading wire of said electric fusehead to another common conducting element connectable to the other pole of a source of electricity, wherein one lead wire of the next detonator to be fired is connected to the common conducting element connectable to the first mentioned pole of the source of electricity and the other lead wire to a leading wire of the electric fusehead of a second electric switch and wherein also the leading wire of one of the terminals of the first electric switch is connected to the electric fusehead of the second electric switch and the leading wire of the other terminal of the first electric switch to the common conducting element connectable to the other pole of a source of electricity, and so on.

More specifically, a layout according to another em bodiment of the invention for firing a succession of shots by the firing of their corresponding electric detonators with a delay period between successive shots comprises electrically actuable delay switches each comprising a container having within it an electric fusehead comprising an electric resistance wire and combustible composition element adapted to ignite to yield hot gases when an electric current is passed through said Wire and two terminals at least one of which is insulated from the container and so distanced from each other and the fusehead that on combustion of said combustible composition the space between the terminals becomes a conducting path for the difference of potential available between the terminals, is one wherein one lead wire of the first detonator to be fired is connected to a common conducting element connectable to one pole of a source of electricity and the other lead wire is connected to a leading wire of the electric fusehead of an electrically actuable delay switch and the other leading wire of the electric fusehead to another common conducting element connectable to the other pole of the source of electricity, wherein one leading wire of the terminals of said switch is connected to one of said common conducting elements and the leading wire of the other terminal of said switch is connected to a lead wire of the second detonator to be fired, and wherein the other lead wire of the second detonator to be fired is connected to a leading wire of the electric fusehead of the second electrically actuable delay switch to be fired and the other leading wire of the electric fusehead of said second delay switch is connected to the other common conducting element and so on.

Preferably instantaneous electric detonators are used in the carrying out of the invention but delay electric detonators can be used if it is desired to control the number of detonators to be initiated before any particular one is allowed to fire.

An assembly according to one embodiment of the invention comprises two common conducting elements each connectable to a source of electricity and a number of spaced electrically actuable delay switches each switch comprising a container having within it an electric fusehead comprising an electric resistance wire and combustible composition element adapted to ignite to yield hot gases when an electric current is passed through said wire and two terminals at least one of which is insulated from the container and so distanced from each other and the fusehead that on combustion of said combustible composition the space between the terminals becomes a conducting path for the diiference of potential available between the terminals, wherein one common conducting element has a number of spaced accessible portions each for electrical connection to one lead wire of a detonator, wherein one leading wire of the electric fusehead of the first electrically actuable delay switch to be fired is connected to the other common conducting element and the other lead wire is free for electrical connection to the other lead wire of the first detonator to be fired, wherein one circuit making terminal of said first delay switch is electrically connected to said second common conducting element and the other circuit making terminal of said first delay switch is electrically connected to one of the leading wires of the second electrically actuable delay switch to be fired, and wherein the other leading wire of the electric fusehead of said second electrically actuable delay switch is free for electrical connection to the other leading wire of the second detonator to be fired, and so on.

An assembly according to another embodiment of the invention comprises two conducting elements each connectable to a pole of a source of electricity and a number of spaced electrically actuable delay switches each switch comprising a container having within it an electric fusehead comprising an electric resistance wire and combustible composition element adapted to ignite to yield hot gases when an electric current is passed through said wire and two terminals at least one of which is insulated from the container and so distanced from each other and the fusehead that on combustion of said combustible composition the space between the terminals becomes .a conducting path for the ditference of potential available between the terminals, wherein one leading Wire of the electric fusehead of the first electrically actuable delay switch to be fired is connected to one of said conducting elements and the other leading wire of said electric fusehead is free for connection to a lead wire of the first detonator to be fired, wherein said other conducting element has an accessible portion for electrical connection to the other lead wire of the first detonator to be fired, wherein one circuit making terminal of said first delay switch is electrically connected to one of said conducting elements and the other circuit making terminal of said first delay switch is free for connection to a lead wire of the second detonator to be fired, and wherein one leading wire of the electric tusehead of the second electrically actuable delay switch to be fired is electrically connected to the other of said conducting elements and the other leading wire of the said electric fusehead of said second electrically actuable delay switch is free for electrical connection to the other leading wire of the second detonator to be tired, and so on.

The electrically actuable delay switch according to the invention comprises a container having within it an electric fusehead comprising an electric resistance wire and combustible composition element adapted to ignite to yield hot gases when an electric current is passed through said wire and two terminals at least one of which is insulated *6 from the container and so distanced from each other and the fusehead that on combustion of said combustible composition the space between the terminals becomes a conducting path for the difference of potential available between the terminals.

The said container may be tubular in form and may be made either of a metal such as copper, aluminum or an alloy of one or both of these or of a non-conducting material, for example a synthetic resin. It is preferable that the two terminals in open circuit should be accommodated near to one another at the end remote from the electric fusehead but in close proximity to it. With a metal tube this can conveniently be done by embedding them and leads thereto in a plug of insulating material around which the metal can be crimpcd so that the electrodes protrude inwardly from the plug and only the leads project outwardly. The electric fusehead can be similarly accommodated in a plug of insulating material around which the tube is crimped at the other end.

The source of supply of electricity used in the improved method of firing a series of explosive charges according to the invention should be such as will give a sustained supply of adequate voltage to bridge the gap between the terminals when the gap becomes conducting and may be either alternating or direct current.

The delay period between the time the actuating current for the electric fusehead commences to pass and that when the space between the terminals becomes conducting can be varied in a number of ways, for example by varying the amount or nature of the combustible composition; by varying the dimensions of the tubular body; by varying the pressure of air or oxygen in the tubular body, or by inserting between the electric fusehead and the terminals a delay fuse element comprising a core of a combustible composition compressed into a tube of non-combustible material, for example metal, fixed in the tubular body in such manner that one end of the core of combustible material is in ignitive contact with the electric fusehead so that when the whole length of the core has burned the gases will pass to the space containing the terminals, render the space conducting and thus permit closure of the open circuit. If desired the two spaced terminals may be embedded in a head of combustible composition and the delay period may be varied by altering the amount or formula of this composition. The delay periods of the individual switches in a series may be the same or different depending on the time interval at which it is desired to fire the individual shots in the round. The delay periods between shots may be further varied by using delay detonators of differing delay periods for some or all of the shots.

By Way of example a portion of a detonator firing circuit for short period delay blasting in accordance with one embodiment of the invention, showing two instantaneous electric detonators and their associated electrically actuable delay switches out of a considerably greater number is illustrated in the diagrammatic drawing accom panying the provisional specification, now referred to as Figure 1.

In Figure 1, 1 is a wire connection to the positive pole of a source of electric current and 2 is a wire connection to the negative pole thereof. 3 is an electrically actuable delay switch in which one lead wire of the fusehead 4 is connected to the line 1 and the other lead wire is connected through the fusehead 5 of the detonator 6 to the line 2. The delay switch 3 is provided with two terminals 7 one of which is connected to the line 1 and the other is connected through the fusehead 4' of a similar switch 3 and the fiusehead 5' of a detonator 6 to the line 2. The terminals 7 are surrounded by a head of combustible material 8. The delay switch 3 is provided with two terminals 7 which are surrounded by a head of combustible material 8' and which can be connected up similarly to continue the series. Thus when the firing switch (not shown) is closed current passes through the fuseheads and 4 causing the detonator 6 to fire and the electrically actuable delay switch 3, to close. Current then passes through fiuseheads 5 and 4' causing the detonator 6' to fire and the electrically actuable delay switch 3 to close and so on through the series of detonators and electrically actuable delay switches. It will be apparent from the layout illustrated that it is impossible for any detonator in such a series to fire before any other earlier detonator in the series. Furthermore, should any switch fail to close satisfactorily none of the subsequent detonators in the series will fire. In practice the leads from the fuseheads 4, 4 etc. to the firing line 1 may be of considerable length whereas the distance between the detonators and their corresponding electrically actuable delay switches may be made so small that the switches may suffer damage or destruction by blast from the explosive charges initiated by their respective detonators' Nevertheless this cannot occur before the fusehead of the corresponding electrically actuable delay switch is fired. As a result a gas generating decomposition will occur in it which will make the space between the terminals conducting. This will lead to the firing of the next detonator.

By way of example a portion of a detonator firing circuit for short period delay blasting in accordance with another embodiment of the invention showing instantaneous electric detonators out of a considerably greater number of detonators and switches is illustrated diagrammatically in Fig. 2 of the diagrammatic drawing attached hereto. Fig.2 also illustrates the assembly comprising three associated electrically actuable delay switches out of a considerably greater number of delay switches electrically connected to the insulated twin twisted wires 1 and 2 and showing the free leading wire of one terminal v of the preceding delay switch and a free leading wire from the electric fusehead of the succeeding delay switch each for subsequent connection to a lead wire of an electric detonator which is to detonate after the firing of said preceding delay switch and is to permit the passage of the applied electric current through the electric fusehead of the succeeding electrically actuable delay switch.

In Fig. 2 one leading wire from the electric fusehead 4 of the electrically actuable delay switch 3' is connected to 1 and the other leading wire from the electric fusehead is shown free but would have been connected to 2 if 3 were the first electric fusehead to be fired in the round. One of the two leading wires from 7, 7' is joined to 2 and the other leading wire from 7, 7 is joined to one of the two lead wires of 5', the other lead wire of 5' 'is joined to one of the two leading wires of 4" and the other leading wire of 4 is joined to 1, and so on. The firing of the matchhead composition of the fusehead 4 in the electrically actuable delay switch 3' leads to the burning of the composition 8 and to the making of the space between the terminals 7', 7' conducting. When this occurs an electric firing current passes from 1 to 2 through 7 7 5' and 4" of the electrically actuable delay switch 3". There is a definite period of delay between the instant at which the fusehead 4" fires and the making of the space between the terminals 7", 7" conducting. This delay period depends inter alia on the burning characteristics of the matchhead composition of the fusehead 4", the gas pressure developed in the switch 3" and the composition of the bead of combustible material 8", so that the firing circuit through the fusehead 5" is only completed after the corresponding delay period, and it will be appreciated that similarly the firing of the next instantaneous detonator in the round will only occur after precisely the'same period of delay from the instant at which this second electric detonator fires, and so on.

An electrically actuable delay switch illustrated by way of example in Figure 2 comprising an aluminium tube mm. long and 6.7 mm. diameter, and terminals 7, 7 dipped in a low tension fusehead composition consisting of a collodion coated lead mononitroresorcinate, potassium chlorate and charcoal composition and located in close proximity to the low tension fusehead 4' having an ignitable composition consisting of a collodion coated lead mononitroresorcinate, potassium chlorate and charcoal composition and which evolves approximately 7 ml. of gas at N. T. P. gives a delay time of approximately 15 milliseconds.

What we claim is:

An electrically actuable delay switch comprising a container having within it an electric fusehead comprising an electric resistance wire and combustible composition element adaptedto ignite to yield hot gases when an electric current is passed through said wire and two terminals at least one of which is insulated from the container and so distanced from each other and the fusehead that on combustion of said combustible composition the space between the terminals becomes a conducting path for the difference of potential available between the terminals.

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